“He is honest and dependable,” replied Shirley, loyally.

“Yes, but I wonder why professional detectives are so primitive.  They wear their calling cards and their business shingles on their figures and faces.  Surely the crooks must know them all personally.  I read detective stories, in rest moments, and every one of the sleuths lives in some well-known apartment, or on a prominent street.  Some day we may read of one who is truly in secret service, but not until after his death notice.  But there, I am talking to quiet my own nerves a bit,—­now we will get to cases.”

The doctor dropped his cigar into the bronze tray on the table, leaning forward with intense earnestness, as he continued.

“This, Mr. Shirley, is the third murder of the sort within a week.  Wellington Serral, the wealthy broker, came to a sudden death in a private dining room last Monday, in the company of a young show girl.  He was a patient of mine, and I signed the death certificate as heart failure, to save the honorable family name for his two orphaned daughters.

“Herbert de Cleyster, the railroad magnate, died similarly in a taxicab on Thursday.  He was also one of my patients.  There, too, was concerned another of these wretched chorus girls.  To-night the fatal number of the triad was consummated in this cycle of crime.  To maintain my loyalty to my patients I have risked my professional reputation.  Have I done wrong?”

“No!  The criminal shall be brought to justice,” replied Shirley in a voice vibrant with a profound determination which was not lost upon his companion.

“Are you powerful enough to bring this about, without disgracing me or betraying this sordid tragedy to the morbid scandal-rakers of the papers?”

“I will devote every waking hour to it.  But, like you, my efforts must remain entirely secret.  I vow to find this man before I sleep again!”

“You are determined—­yet it cannot be one single man.  It must be an organized gang, for all the crimes have been so strangely similar, occurring to three men who are friends, and entrez nous, notorious for their peccadilloes.  The girls must be in the vicious circle, and ably assisted.  But there is one thing I forgot to tell you, which you forgot to ask.”

“And this is?”

“How they died.  It was by some curious method of sudden arterial stoppage.  Old as they were, some fiendish trick was employed so skilfully that the result was actual heart failure.  There was no trace of drugs in lungs or blood.  On each man’s breast, beneath the sternum bone I found a dull, barely discernible bruise mark, which I later removed by a simple massage of the spot!”

Shirley closed his eyes, and passed his hand over his own chest —­along the armpits—­behind his ears—­he seemed to be mentally enumerating some list of nerve centers.  The physician observed him curiously.

“I have it, doctor!  The sen-si-yao!”